coal ball

A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab. Coal balls were formed by the early permineralisation of peat by calcite in Carboniferous Period swamps and mires before the peat could . As such, despite not actually being made of coal, the coal ball owes its name to its similar origins as well as its similar shape with actual coal.